Though, I think there's still room for improvement. Currently the thumbnail generation for MP3 files seem oddly slow when compared to the speed it takes to generate the thumbnails when 'recognize only by extension' is disabled, but it's a minor annoyance with thumbnail cache on.

ckv wrote:Currently the thumbnail generation for MP3 files seem oddly slow when compared to the speed it takes to generate the thumbnails when 'recognize only by extension' is disabled, but it's a minor annoyance with thumbnail cache on.

It seems that XnView is combing through the whole MP3 file looking for start & end signatures of a (JPEG or PNG) picture stream. During this process, the parsed data is dumped into the system's %TEMP% folder. That's why we initially had “empty temp files left behind when browsing”.